Cemented carbides, e.g., cobalt cemented tungsten carbide, have been used in a variety of non-cutting tool applications where the wear resistance, high elastic modulus, compressive strength, resistance to fracture, or any combination of the preceding provide a component with a long lifetime under conditions involving high temperature, pressure, or both in various environments. However, when these components are placed within a corrosive environment, the expected lifetime of the cemented carbide component can be significantly reduced. This can be of great concern when the cemented carbide components involved are (1) large and, therefore expensive; (2) used in equipment or a process where failure during use can cause significant damage; or (3) both.
For example, cobalt cemented tungsten carbide plungers have been used in hyper compressors used to produce the high gas pressures, for example, up to about 344 megapascal (MPa) (50,000 pounds per square inch (psi)). These high pressures as well as temperatures up to about 330.degree. C. (626.degree. F.) are required during the manufacture of materials such as low density polyethylene (LDPE). The high modulus of elasticity and resistance to buckling, deformation, fracture and wear of cobalt cemented tungsten carbide alloys, such as "K94.TM." cobalt cemented tungsten carbide or "KZ94.TM." cobalt cemented tungsten carbide, under these conditions, are responsible for the commercial success of cemented carbides in these applications ("Properties and Proven Uses of Kennametal Hard Carbide Alloys," Kennametal Inc. (1977) Pages 1-48). This success comes despite the cost of manufacturing and the degree of care required in handling, using, and maintaining plungers made of cemented carbides ("Care and Handling of Tungsten Carbide Plungers for Hyper Compressors," Kennametal Inc. (1978) Pages 1-12).
To truly appreciate the present invention, one must realize the degree of care required in manufacturing, handling, using, and maintaining plungers made of cemented carbides. In addition to possessing the appropriate mechanical and physical properties, a plunger is manufactured to exacting tolerances, with a typical surface finish of about 0.025 micrometer (one microinch) or better--a mirror-like finish. During handling and storage outside of a hyper compressor and use or while sitting idle in a hyper compressor, in addition to the wear a plunger experiences during use, the cemented carbide comprising a plunger is also subject to corrosion or leaching of binder (e.g., cobalt). This corrosion may affect the lifetime of the plunger. For example, during use corroded or leached areas can experience local frictional heating which induces heat stress cracking of the area. These difficulties are typically addressed by periodically dressing (e.g., grinding, honing, repolishing, or any combination of the preceding) the entire surface of a plunger to not only remove the corroded or leached areas from the surface but also reduce a plunger's diameter. The dressing of a plunger may be repeated until the diameter has been so reduced that a the plunger can no longer be used to pressurize a hyper compressor. In addition to localized frictional heating, corroded or leached areas also create stress intensifiers that effectively reduce the load bearing ability of a cemented carbide to the point that a plunger may fail during use.
During handling and storage, the corrosion or leaching of the binder from a commercially available cemented carbide plunger may be readily minimized by following prescribed practices. Furthermore, these commercially available cemented carbides have historically exhibited suitable corrosion resistant properties when used in hyper compressors to manufacture low density polyethylene (LDPE).
In recent years, however, the low density polyethylene industry has been developing improved low density polyethylene and copolymers of polyethylene. In addition to the traditional feedstock ingredients, such as initiators (e.g., oxygen, peroxides or azo compounds), chain transfer agents (e.g., alcohols, ketones, or esters), or both the most recent additional ingredients to the feedstock stream of a hyper compressor create a extremely aggressive environment that corrodes, leaches, or both the binder of commercially available cemented carbides.
For the forgoing reasons there is a need for a cermet composition possessing at least equivalent mechanical properties, physical properties, or both of currently used materials while possessing superior corrosion resistance in comparison to currently used materials in applications involving, for example, high temperature, pressure, or both and that can be easily manufactured.